1. Field of the Invention
The present invention relates generally to apparatus for measuring downhole the properties of a hydrocarbon-bearing fluid, and more particularly, but not by way of limitation, to an apparatus capable of making multiple downhole measurements without being removed from the well.
2. Brief Description of the Prior Art
It is often desirable in well testing operations to retrieve downhole fluid samples for inspection and analysis. One portion of the lab analysis which is usually performed on a sample is a determination of the physical properties of a sample. One of the properties which must be determined is the pressure below which the gas present in an oil sample will begin to leave the single-phase oil sample and break out of solution, creating a two-phase oil and gas sample.
This pressure is dependent on the temperature of the sample, and is known as the bubble-point pressure. The bubble-point pressure of a sample is determined by placing the sample into a laboratory cell where the heat and pressure can be controlled. In some cases, a crude bubble-point determination is made at the well site, at ambient conditions. The determined value of the bubble point at ambient conditions is then used to extrapolate or predict the bubble point at downhole temperatures.
In any case, the goal is usually to determine the bubble point for the sample at one or more downhole locations by estimating what happens to the sample properties under these conditions or by trying to recreate the downhole conditions.
One problem with present methods of determining bubble point pressure and other sample properties is with the handling of the sample. The sample is captured downhole, then retrieved to the surface. As the sample is retrieved to surface, it cools down and the sample pressure drops. In some samples, irreversible chemical changes occur as the sample cools down. This problem is unavoidable with present methods.
Another problem which can arise with present sampling techniques, is sample degradation due to long-term storage at ambient conditions. It often takes a long time for samples to be shipped from the well site to a lab for analysis.
Human error in the processes described above can also result in sample corruption, contamination or degradation in some other way. Unclean sample bottles, lab equipment, etc., can also degrade the sample.
Thus there is a need for an apparatus which can readily determine the bubble-point pressure and other desired parameters of a well fluid sample downhole within the well while the sample is still at its natural conditions.